Optical transmission systems often use an optical service channel to communicate status and control information between various transceivers, amplifiers and transponders in an optical transmission system. It is important to minimize the insertion loss of the wavelength multiplexing filter used to couple the optical channels used to transmit payload data such as wave division multiplex (WDM) channels and the optical service channel (OSC).
Several prior art approaches exist, but none have the features of the current invention. For instance, U.S. Pat. No. 6,327,060 to Otani discloses an optical transmission system having an add drop station controlled by four optical circulators which allows signals to be added and dropped via fiber gratings which reflect selective wavelengths. The invention accomplishes a bypass of an optical supervisory channel but does not provide for the insertion and removal of an optical supervisory channel by optical circulators. Otani also suffers from adding additional unnecessary optical components which increase optical losses.
Another example is U.S. Pat. No. 6,122,095 to Fitehi. This patent discloses an optical add/drop multiplexor using one or more fiber gratings which are disposed along the length of rare earth doped fiber or between segments for reflecting optical signals which are added or dropped through circulators. However, Fitehi does not provide for a separate counter propagating optical service channel.
Another example is U.S. Pat. No. 5,299,048 to Suyama. This patent provides an optical communication system which employed a dichromic separator to distinguish between a signal light and a pumping light where the pumping light carries control information. However, Suyama suffers from the addition of losses in the dichromic separator and other losses associated with the addition of other optical components.
Therefore, a need exists for an optical transmission system which has an additional optical service channel for system management which has minimal impact on the WDM channels in the area of insertion loss.